Drag Race 101: Headers, Part II
In Part I, we looked at header primary tube lengths (equal and not so equal). We also examined inertial scavenging. Basically, getting the tube length right is important. It’s an interesting topic, but as some have pointed out, why can’t you simply use a flow bench come up with the right header primary tube size? Obviously, tube size has an effect upon inertial scavenging. With all other factors being equal, you will obtain a superior scavenging effect with an increase in exhaust speed. Some folks will even tell you that the exhaust valve diameter determines the header tube diameter (not exactly correct).
Now, to a point, a header manufacturer can control the speed of the exhaust gas by changing the diameter of the primary tube. The smaller the diameter of the tube, the faster the exhaust will flow (keeping in mind the exhaust flow slows as it cools). A clever header builder will recognize that by varying both the length and the diameter of a given primary tube in a header, those tubes can be tuned to provide the largest amount of inertial scavenging. But there’s something else that is equally important and it can have an effect upon the header configuration, and it also shows why a flow bench is pretty much useless when dealing with header sizing:
Wave Scavenging
Wave scavenging does not involve the physical movement of exhaust gases up and down the header tubes. Instead, it deals with the sound waves created in the engine. When an exhaust valve first opens, a pressure wave begins to travel — moving in excess of 18,500 inches per second (more than the speed of sound, which at sea level is 13,514 in/sec). This sonic wave is moving quickly – much quicker than the ever expanding exhaust wave. When the sonic wave arrives at the end of the primary tube, a negative shock wave is generated, and this wave travels backwards toward the exhaust port (that means it is “reflected”). What the header builder has to do is to time the arrival of this negative pressure wave to occur just before the exhaust valve has closed and while the intake valve is opening. If the negative wave arrives too soon or too late, the power potential of the next combustion chamber cycle will be “diluted” significantly. Obviously, a flow bench doesn’t have this capability.
Generally speaking the engine likes to see a lower pressure and a higher speed in the exhaust (the larger the tube, the lower the exhaust speed). Typically, a situation with a highly negative pressure in the exhaust during the camshaft overlap period (when both inlet and exhaust valves are open) will produce power.
Equally important, an accurate set of specifications for a “tuned” exhaust system can’t be created easily by way of mathematics either. Certainly there are formulas and software available (…readily available, and in an upcoming segment, we’ll provide you with examples) to provide basic header dimensions, but many experts will tell you that in order to accurately arrive at the exact header primary tube length and diameter, there is only one real option: Trial and error.
Collector Length or Primary Length – What’s Important?
Collector length is important, but some racers tend to ignore it. Sure dozens upon dozens of sharp little guy racers take the time to adjust collector length for their given combinations. But the problem is, some racers think collector length is directly related to peak engine horsepower. This could very well be wrong.
Collector length is determined, primarily, by the variables of the racecar in question, and is not entirely dependent upon the engine alone. That means the header data you collect from an engine dyno test could very well prove invalid when the motor is installed in the car. When tuning the headers (and the entire exhaust system for that matter) for a given combination, there is a huge cross section of variables. Things such as vehicle weight, rear axle ratio, transmission type and end use of the vehicle all come into play. Once the variables are all taken into account, the header manufacturer can then determine at what RPM level the peak torque curve should be developed. In contrast to what may have been discussed and written in the past (and perhaps incorrectly), the most important thing to consider when selecting headers is the primary tube diameter. It’s something you can’t change without buying new headers. But you have to remember that primary tube length can be used to move the torque curve in either direction (effectively moving the torque curve from one RPM level to another).
We’ll dig deeper into collectors in Part III.
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So You Want to Be A Drag Racer: Buying, Building, Wrenching and Racing
The allure of the drag strip is easy to understand – a place where it takes less than 10 seconds to make a stand, prove your skill, speed, and nerve. But the road to the races can be intimidating. The Burnout wants to make that road a lot smoother for aspiring racers, whether it’s through building a new car, modding a used one, or taking that ride all the way up to the burnout box and beyond. This series is a work in progress, an ever expanding comprehensive guide to all the things that take drag racing from concept to reality.
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